1. Field of the Invention
The invention relates to batch asphalt plants and, more particularly, relates to a batch asphalt plant capable of producing batches of hot mix asphalt (HMA) each having a percentage of recycled asphalt products (RAP).
2. Discussion of the Related Art
Batch asphalt plants are well-known for the production of discrete amounts or batches of HMA. Such plants typically include a drum dryer for heating and drying stone aggregate, a tower, and an enclosed vertical bucket elevator for conveying the heated and dried aggregate from the dryer onto vibrating deck screens which are mounted on top of the tower and which segregate the aggregate by size. Chutes direct the segregated aggregate to bins which underlie the screens such that each bin receives an aggregate of a predetermined average size. A weigh hopper is positioned below the bins and permits the aggregate from one or more bins to be weighed out to provide a designated amount of a desired mixture. The characteristics of such mixtures may vary, e.g. from a roadway base coat to a finish coat. A pugmill is positioned beneath the weigh hopper for mixing the aggregate with liquid asphalt. HMA is dumped into an underlying truck from a gate located at the bottom of the pugmill, and vapors are evacuated from the top of the pugmill and conveyed to a baghouse by a scavenging system.
The use of recycled asphalt product (RAP) is often desirable in the manufacturer of HMA. Two methods of adding RAP to HMA in batch plants have heretofore been employed.
The first method involves the addition of limited percentages of RAP to the virgin aggregate in the vertical bucket elevator conveying virgin aggregate from the dryer to the tower. This method, though relatively simple, is poorly suited for use with batch asphalt plants for several reasons. For example, only limited amounts of RAP can be added to the virgin aggregate without clogging the deck screens above the bins or otherwise interfering with the flow of materials into or from the bins. In addition, there is no way of varying the percentage of RAP in a given HMA batch since each batch of RAP is disbursed throughout the material in the bins and since not all bins are used to supply material to every batch.
The second method of adding RAP to HMA in batch plants involves the direct feed of RAP to the pugmill. This second method permits the use of higher percentages of RAP since there is no danger of clogging screens or otherwise interfering with the flow of materials into downstream components of the system. This method also permits the percentage of RAP to be varied from batch to batch since all of the RAP introduced into the pugmill is in each instance mixed directly with the final product. However, this method exhibits a rather severe drawback in that, when the RAP is added to the hot virgin aggregate in the pugmill, a virtual explosion of steam and dust results from the rapid vaporization of the moisture in the RAP. The typical pugmill scavenging system is designed to handle only the average production of dust and steam from a normal mixing process and thus may well be overwhelmed by the explosive vaporization of moisture from the RAP. Uncontrolled introduction of RAP into the pugmill could thus result in overloading of the scavenging system and the release of potentially harmful pollutants to the atmosphere. This problem could heretofore be avoided only by grossly oversizing the capacity of the scavenging system, by drying the RAP prior to its introduction into the pugmill, or by limiting the percentages of RAP to limit steam production to acceptable levels. Each of these techniques is undesirable for cost and/or operational reasons. The need has therefore arisen to provide a batch asphalt plant capable of mixing a desired quantity of RAP in each production batch in a more desirable manner.
The traditional scavenging process exhibits several drawbacks and disadvantages. For instance, both the gases and particulates conveyed to the baghouse by the scavenging system have relatively high levels of hydrocarbon concentrations. These hydrocarbons could condense in the baghouse and cause a fire hazard. The need has therefore arisen to handle safely materials scavenged from the pugmill of a batch asphalt plant.
Another problem associated with conventional batch asphalt plants is the inability of such plants to adequately handle materials scavenged from the pugmill by the scavenging system. These materials typically include a mixture of steam, gases, and dust which are conveyed via a ductwork assembly of the scavenging system from the pugmill to a baghouse where the particulates are separated from the gases.